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# merkata's solution

## to Rail Fence Cipher in the Go Track

Published at May 04 2021 · 0 comments
Instructions
Test suite
Solution

Implement encoding and decoding for the rail fence cipher.

The Rail Fence cipher is a form of transposition cipher that gets its name from the way in which it's encoded. It was already used by the ancient Greeks.

In the Rail Fence cipher, the message is written downwards on successive "rails" of an imaginary fence, then moving up when we get to the bottom (like a zig-zag). Finally the message is then read off in rows.

For example, using three "rails" and the message "WE ARE DISCOVERED FLEE AT ONCE", the cipherer writes out:

``````W . . . E . . . C . . . R . . . L . . . T . . . E
. E . R . D . S . O . E . E . F . E . A . O . C .
. . A . . . I . . . V . . . D . . . E . . . N . .
``````

``````WECRLTEERDSOEEFEAOCAIVDEN
``````

To decrypt a message you take the zig-zag shape and fill the ciphertext along the rows.

``````? . . . ? . . . ? . . . ? . . . ? . . . ? . . . ?
. ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? .
. . ? . . . ? . . . ? . . . ? . . . ? . . . ? . .
``````

The first row has seven spots that can be filled with "WECRLTE".

``````W . . . E . . . C . . . R . . . L . . . T . . . E
. ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? .
. . ? . . . ? . . . ? . . . ? . . . ? . . . ? . .
``````

Now the 2nd row takes "ERDSOEEFEAOC".

``````W . . . E . . . C . . . R . . . L . . . T . . . E
. E . R . D . S . O . E . E . F . E . A . O . C .
. . ? . . . ? . . . ? . . . ? . . . ? . . . ? . .
``````

Leaving "AIVDEN" for the last row.

``````W . . . E . . . C . . . R . . . L . . . T . . . E
. E . R . D . S . O . E . E . F . E . A . O . C .
. . A . . . I . . . V . . . D . . . E . . . N . .
``````

If you now read along the zig-zag shape you can read the original message.

## Coding the solution

Look for a stub file having the name rail_fence_cipher.go and place your solution code in that file.

## Running the tests

To run the tests run the command `go test` from within the exercise directory.

If the test suite contains benchmarks, you can run these with the `--bench` and `--benchmem` flags:

``````go test -v --bench . --benchmem
``````

Keep in mind that each reviewer will run benchmarks on a different machine, with different specs, so the results from these benchmark tests may vary.

## Further information

For more detailed information about the Go track, including how to get help if you're having trouble, please visit the exercism.io Go language page.

## Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

### cases_test.go

``````package railfence

// Source: exercism/problem-specifications
// Commit: 88db37b rail-fence-cipher: apply "input" policy
// Problem Specifications Version: 1.1.0

type testCase struct {
description string
message     string
rails       int
expected    string
}

// encode
var encodeTests = []testCase{

{"encode with two rails",
"XOXOXOXOXOXOXOXOXO",
2,
"XXXXXXXXXOOOOOOOOO"},

{"encode with three rails",
"WEAREDISCOVEREDFLEEATONCE",
3,
"WECRLTEERDSOEEFEAOCAIVDEN"},

{"encode with ending in the middle",
"EXERCISES",
4,
"ESXIEECSR"},
}

// decode
var decodeTests = []testCase{

{"decode with three rails",
"TEITELHDVLSNHDTISEIIEA",
3,
"THEDEVILISINTHEDETAILS"},

{"decode with five rails",
"EIEXMSMESAORIWSCE",
5,
"EXERCISMISAWESOME"},

{"decode with six rails",
"133714114238148966225439541018335470986172518171757571896261",
6,
"112358132134558914423337761098715972584418167651094617711286"},
}``````

### rail_fence_cipher_test.go

``````package railfence

import "testing"

func testCases(op func(string, int) string, cases []testCase, t *testing.T) {
for _, tc := range cases {
if actual := op(tc.message, tc.rails); actual != tc.expected {
t.Fatalf("FAIL: %s\nExpected: %q\nActual: %q", tc.description, tc.expected, actual)
}
t.Logf("PASS: %s", tc.description)
}
}

func TestEncode(t *testing.T) { testCases(Encode, encodeTests, t) }
func TestDecode(t *testing.T) { testCases(Decode, decodeTests, t) }``````
``````package railfence

// Encode spirals down and up a rail to encode a message
// "HELLOGO" in two rails would be "HLOOELG" because:
// H L O O index 0,2,4,6
//  E L G  index 1,3,5
// With three rails and above one needs to consider that
// we don't go down but up as well (when indexing our array)
// "HELLOGO" in three rails will thus be "HOELGLO":
// H   O    index 0,4
//  E L G   index 1,3,5
//   L   O  index 2,6
// Note how between every regular step (for 3 rails it is a step of 4)
// we insert an additional char somewhere in between if we are not
// in the first or last rail - this is expressed as middleStep in the code
func Encode(message string, rails int) string {

return railf("encode", message, rails)
}

func Decode(message string, rails int) string {

return railf("decode", message, rails)
}

// encoding and decoding differ only in the positions for reading and writing
// to avoid duplication, the logic is placed in a helper and a switch statement
// is differentiating between the desired operations - encode and decode
func railf(op, message string, rails int) string {
result := []byte(message)

pos := 0
for i := 0; i < rails; i++ {
for j := i; j < len(message); j += rails + (rails - 2) {
if j >= len(message) {
break
}
// middle rails get double the chars
// there is an intermediate char
// at (rails + (rails - 2)) - 2*i distance
// between each regular step interval
switch op {
case "encode":
result[pos] = message[j]
case "decode":
result[j] = message[pos]
}
pos++
if i > 0 && i < rails-1 {
middleStep := (rails + (rails - 2)) - 2*i
if j+middleStep < len(message) {
switch op {
case "encode":
result[pos] = message[j+middleStep]
case "decode":
result[j+middleStep] = message[pos]
}
pos++
}
}
}
}

return string(result)
}``````